The present invention relates to a telephone subscriber accommodation system in a broadband network, and specifically to a technique for accommodating telephone subscribers in a broadband network where information is transmitted in a plurality of cells in an asynchronous transfer mode (ATM).
A broadband network (B-ISDN) transmits information in a plurality of cells in an asynchronous transfer mode (ATM), and features a unique method using the same network and the same signaling process in transmitting various kinds of information including broadband information such as data images, narrowband information as used for telephone communication, and burst information as used for data transmission. In this broadband network, user information is transmitted in fixed-length cells 1 whose structure is shown in FIG. 1, and the transmission speed can be shifted by modifying the number of cells assigned to one communication in a given unit of time. In this method, cell 1 comprises a 5-octet header part 1a and a 48-octet information field 1b.
In this broadband network, a communication must be conducted optionally between a telephone subscriber accommodated in the broadband network and a telephone subscriber accommodated in an existing telephone network. FIG. 2 shows a configuration in which a telephone subscriber is accommodated in a broadband network (B-ISDN). 11 is a broadband network. 12 is an existing telephone network. 13 is an adaptor (ADP) for connecting the broadband network to the existing telephone network. 14 is a telephone unit on the side of the broadband network (hereinafter referred to as a broadband subscriber). 15 is a terminal adaptor (TA) for converting voice data to transmit them in cells. 16 is a network terminator (NT1). 17 is a subscriber line. 18 and 19 are telephone units connected to an existing telephone network (hereinafter referred to as existing subscribers).
For a telephone subscriber, communication data are transmitted as 64 Kbps continuous data, and must be transmitted with only short delay.
The following two prior art technologies indicate how to transmit voice data, that is, communication data, in cells.
In the first prior art technology, voice data are put in a cell at a rate of 8 bits every 125 .mu.s (a sampling cycle of voice data) and transmitted to the broadband network 11.
In the second prior art technology, voice data are divided into data of the size of the information field 1b (48.times.8 bits) of a cell before they are put into cells (refer to FIG. 1). Then they are transmitted in cells by the broadband network 11. That is, the second prior art has a configuration similar to that of a voice packet which is transmitted in a packet network.
In the above described first prior art technology, voice data are sent to the broadband network after being put in a cell at the rate of 8 bits every 125 .mu.s in the terminal adaptor 15. That is, only 8 bits are used in the information field 1b of a cell (refer to FIG. 1), and the remaining 47.times.8 bits are transmitted as unoccupied. On receipt of a cell, the adaptor 13 (ADP)(refer to FIG. 2), which is an interface for the existing telephone network 12, converts a cell in a subscriber's transmission form in the existing telephone network. The converted voice data are accommodated in a circuit equivalent to a subscriber interface circuit in the existing telephone network 12, and processed as a subscriber in this network.
In the first prior art technology described above, the delay time during the transmission of information in the broadband network 11 is kept short, and the terminal adaptor 15 only has to transmit to the broadband network after putting voice data in a cell at the rate of 8 bits every 125 .mu.s (or process in the reverse order). Therefore, the configuration is very simple. However, only 8 bits are used in an information field of a cell, and the remaining 47.times.8 bits are reversed as unocuppied. Therefore, an actual band in use in the communication line in the broadband network is not equal to 64 Kbps, but amounts to 64 Kbps.times.48=2.87 Kbps. This is the problem of the first prior art technology.
In the second prior art technology, voice data are transmitted to the broadband network 11 after being put in cells in 48.times.8-bit units by a terminal adaptor. Cells that are transmitted and converted in the broadband network 11 are converted again in the transmission form of the existing telephone network subscriber in the adaptor 13, interfaced for the existing telephone network 12 as described in the first prior art technology, and processed as a subscriber in the existing telephone network 12.
However, in the above described second prior art technology, although the occupied band in the communication line in the broadband network 11 amounts to only 64 Kbps, a buffer is required for the adaptor 13 and the terminal adaptor 15, thus complicating its configuration and causing a problem that the delay may be extended.
Furthermore, in the above described first and second prior art technologies, a user has to use a circuit, as an interface for the existing telephone network 12, similar to that used by subscribers of the existing telephone network. Therefore, the capacity of a switching unit in the existing telephone network is enlarged in proportion to the increasing number of subscribers to be provided with services.